1. Field of the Invention
The present invention relates to satellite navigation and positioning systems. More particularly, the present invention relates to a pseudo-satellite (pseudolite) navigation system which enhances operation of a global navigational satellite system (GNSS) when satellite ephemeris data is unavailable due to jamming or otherwise too weak.
2. Description of the Related Art
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In this regard, the present invention will be discussed with respect to the global positioning system (GPS). However, it will be apparent to those skilled in the art that the present invention can be applied to any global navigational satellite system (GNSS), including the Russian global orbiting navigational satellite system (GLONASS). The present invention is not intended to be limited to GPS applications.
The use of GPS signals to calculate position and GPS time is known in the art. There are times when it is difficult or impossible to receive the true GPS signals from the satellites. For example, there are times when the GPS signals are jammed in a localized area, or a GPS receiver may be located in a position, e.g. a valley, where it cannot receive signals from a sufficient number of satellites.
In order to make GPS signals available to receivers in locations where the true signal is being jammed, the concept of pseudolites was invented. A pseudolite includes a pseudolite transmitter that sends out pseudolite signals which are designed to simulate the true GPS signals from the satellites. An example of a ground-based pseudolite is disclosed in U.S. Pat. No. 5,886,665. An example of an airborne pseudolite is disclosed in U.S. Pat. No. 5,886,666.
U.S. Pat. No. 6,590,524 discloses a method of controlling a slave time by adjusting the slave time based on a master time. In one embodiment, a global positioning system (GPS) receiver calculates GPS time and sends a correction signal to a pseudolite, in order to slave the pseudolite time to GPS time.
The disclosures of U.S. Pat. Nos. 5,886,665, 5,886,666, and 5,590,524 are hereby incorporated by reference in their entireties.
A major limitation of the current art in pseudolite navigation systems is their limitation of usefulness to line of sight distances. RF navigation signals are attenuated by terrain and the curvature of the earth. Ground transmitters are limited to 30–40 km operational ranges. Airborne transmitters are limited to 300 Km maximum even at an altitude of 65,000 ft. A second limitation is the ease of adversaries to jam or otherwise interfere with known GNSS frequencies. GNSS signals in space are low power and susceptible to small amounts of interference levels.
As will be disclosed below the present invention provides a pseudolite system that can operate beyond the limitations of terrestrial line of sight. This “covert” pseudolite system also benefits the user by not requiring specific dependence on a fixed RF frequency spectrum. Frequencies are determined by the host communications systems and can be arbitrary changed as needed.